Tweak the behavior of jj prev when @ is a non-discardable tip

Before this change, `jj prev` does this:
```
jj log -T 'separate(" ",change_id.short(), description)'
@  umspkqwnpqvx add file2
◉  nprzlkrnkpuu add file1
◉  zzzzzzzzzzzz
jj prev
Working copy now at: otzuvlqz 21a57e68 (empty) (no description set)
Parent commit      : zzzzzzzz 00000000 (empty) (no description set)
Added 0 files, modified 0 files, removed 2 files
jj log -T 'separate(" ",change_id.short(),description)'
@  otzuvlqzsrxt
│ ◉  umspkqwnpqvx add file2
│ ◉  nprzlkrnkpuu add file1
├─╯
◉  zzzzzzzzzzzz
```

After, it does this:
```
jj log -T 'separate(" ",change_id.short(), description)'
@  okxltnqrlpsr
│ ◉  umspkqwnpqvx add file2
├─╯
◉  nprzlkrnkpuu add file1
◉  zzzzzzzzzzzz
```

There is more discussion about this change in
#3426.

This commit also allows `jj prev` to run when `@` is a merge commit. Previously
it refused to do this.

cli/src/commands/prev.rs

@@ -67,7 +67,6 @@ pub(crate) fn cmd_prev(
     args: &PrevArgs,
 ) -> Result<(), CommandError> {
     let mut workspace_command = command.workspace_helper(ui)?;
-    let offset = args.offset;
     let current_wc_id = workspace_command
         .get_wc_commit_id()
         .ok_or_else(|| user_error("This command requires a working copy"))?;
@@ -78,25 +77,17 @@ pub(crate) fn cmd_prev(
             .heads()
             .contains(current_wc_id);
     let current_wc = workspace_command.repo().store().get_commit(current_wc_id)?;
-    let start_id = if edit {
-        current_wc_id
+    let offset = if current_wc.is_discardable() && !args.edit {
+        // If the current working copy is discardable and the user didn't
+        // specify --edit, then we move one commit further back. This is so that
+        // the new empty working commit is created on the parent of the current
+        // on (or further back depending on the value of args.amount). If we
+        // didn't do this, then we wouldn't move far enough.
+        args.offset + 1
     } else {
-        match current_wc.parent_ids() {
-            [parent_id] => parent_id,
-            _ => return Err(user_error("Cannot run `jj prev` on a merge commit")),
-        }
-    };
-    let ancestor_expression = RevsetExpression::commit(start_id.clone()).ancestors_at(offset);
-    let target_revset = if edit {
-        ancestor_expression
-    } else {
-        // Jujutsu will always create a new commit for prev, even where Mercurial cannot
-        // and fails. The decision and all discussion around it are available
-        // here: https://github.com/martinvonz/jj/pull/1200#discussion_r1298623933
-        //
-        // If users ever request erroring out, add `.ancestors()` to the revset below.
-        ancestor_expression.minus(&RevsetExpression::commit(current_wc_id.clone()))
+        args.offset
     };
+    let target_revset = RevsetExpression::commit(current_wc_id.clone()).ancestors_at(offset);
     let targets: Vec<_> = target_revset
         .evaluate_programmatic(workspace_command.repo().as_ref())?
         .iter()
@@ -105,13 +96,11 @@ pub(crate) fn cmd_prev(
     let target = match targets.as_slice() {
         [target] => target,
         [] => {
-            return Err(user_error(format!(
-                "No ancestor found {offset} commit{} back",
-                if offset > 1 { "s" } else { "" }
-            )))
+            return Err(user_error("No ancestor found at requested offset."));
         }
         commits => choose_commit(ui, &workspace_command, "prev", commits)?,
     };
+
     // Generate a short commit hash, to make it readable in the op log.
     let current_short = short_commit_hash(current_wc.id());
     let target_short = short_commit_hash(target.id());

cli/tests/test_next_prev_commands.rs

@@ -112,14 +112,25 @@ fn test_prev_non_discardable_working_copy() {
     let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["prev"]);
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @r###"
-    Working copy now at: royxmykx f04d8595 (empty) (no description set)
-    Parent commit      : qpvuntsm 69542c19 (empty) first
+    Working copy now at: royxmykx 5647d685 (empty) (no description set)
+    Parent commit      : rlvkpnrz 5c52832c (empty) second
+    "###);
+
+    // Since the user didn't specify --edit, @ is now sitting at a new empty
+    // child of second.
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  royxmykxtrkr
+    │ ◉  kkmpptxzrspx third
+    ├─╯
+    ◉  rlvkpnrzqnoo second
+    ◉  qpvuntsmwlqt first
+    ◉  zzzzzzzzzzzz
     "###);
 }
 
 #[test]
 fn test_prev_multiple_without_root() {
-    // Move from fourth => first.
+    // Move from fourth => second.
     let test_env = TestEnvironment::default();
     test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
     let repo_path = test_env.env_root().join("repo");
@@ -139,8 +150,18 @@ fn test_prev_multiple_without_root() {
     let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["prev", "2"]);
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @r###"
-    Working copy now at: vruxwmqv bcea672c (empty) (no description set)
-    Parent commit      : qpvuntsm 69542c19 (empty) first
+    Working copy now at: vruxwmqv 6c3e8d2a (empty) (no description set)
+    Parent commit      : rlvkpnrz 5c52832c (empty) second
+    "###);
+
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  vruxwmqvtpmx
+    │ ◉  zsuskulnrvyr fourth
+    │ ◉  kkmpptxzrspx third
+    ├─╯
+    ◉  rlvkpnrzqnoo second
+    ◉  qpvuntsmwlqt first
+    ◉  zzzzzzzzzzzz
     "###);
 }
 
@@ -257,7 +278,7 @@ fn test_next_choose_branching_child() {
 }
 
 #[test]
-fn test_prev_fails_on_merge_commit() {
+fn test_prev_on_merge_commit() {
     let test_env = TestEnvironment::default();
     test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
     let repo_path = test_env.env_root().join("repo");
@@ -279,9 +300,17 @@ fn test_prev_fails_on_merge_commit() {
     "###);
 
     // Try to advance the working copy commit.
-    let stderr = test_env.jj_cmd_failure(&repo_path, &["prev"]);
+    let (stdout, stderr) = test_env.jj_cmd_stdin_ok(&repo_path, &["prev"], "2\n");
+    insta::assert_snapshot!(stdout,@r###"
+    ambiguous prev commit, choose one to target:
+    1: zsuskuln edad76e9 right | (empty) second
+    2: qpvuntsm 5ae1a6a5 left | (empty) first
+    q: quit the prompt
+    enter the index of the commit you want to target: 
+    "###);
     insta::assert_snapshot!(stderr,@r###"
-    Error: Cannot run `jj prev` on a merge commit
+    Working copy now at: yostqsxw a9de0711 (empty) (no description set)
+    Parent commit      : qpvuntsm 5ae1a6a5 left | (empty) first
     "###);
 }
 
@@ -348,7 +377,7 @@ fn test_prev_onto_root_fails() {
     // The root commit is before "first", which is 5 commits back.
     let stderr = test_env.jj_cmd_failure(&repo_path, &["prev", "5"]);
     insta::assert_snapshot!(stderr,@r###"
-    Error: No ancestor found 5 commits back
+    Error: No ancestor found at requested offset.
     "###);
 }